The formation of solids and emulsions in crude oil during extraction and separation processing presents a plethora of problems that may lead to production delays and/or an inferior sales product(s). These solids and emulsions possess distinct physical and chemical properties. They also tend to originate from crude oils of differing hydrocarbon contents and, once formed, pose their own unique set of problems. For example, the formation of solids may result in:                the slowing or complete cessation of oil flow in pipelines;        plugging of chokes, valves, pumps and vessel internals;        the blocking of water legs in separators;        unplanned shutdowns due to hardened deposits causing blockages;        disposal issues due to the presence of heavy metals;        negative impact on water quality due to an increased oil content in the separated water; and        negative impact on injection/disposal well performance.        
Removal of these solids is often difficult, expensive and potentially hazardous to human health.
The formation of solids precipitation from liquid hydrocarbons such as crude oil generally results from the reaction of metal cations with indigenous naphthenic acids. In this context, naphthenic acids are generally considered to be complex mixtures of alkyl-substituted acyclic and cyclic carboxylic acids that are generated from in-reservoir biodegradation of petroleum hydrocarbons. They are normal constituents of nearly all crude oils and may be present in amounts of up to 4% by weight. The metal cations involved include alkali and alkaline-earth metals such as sodium, potassium, calcium and magnesium. Transition metals such as iron may also be involved. However, most solids normally contain a predominant amount of calcium naphthenate species that are formed from tetraprotic carboxylic acid and/or carboxylate anions and calcium cations. The tetra acids, when they interact with divalent ions, form ionic crosslinks between the multiple acid groups on the tetra acid resulting in a highly crosslinked network. They may precipitate as gummy to hard, solid scale deposits that render control systems inoperable and are detrimental to discharge water and export oil quality.
The calcium naphthenate solids are distinct chemically and physically from the emulsions which are generally associated with sodium carboxylates caused by the reaction of monocarboxylic acids and sodium ions. These are often referred to as carboxylate salts or soaps. They produce flow assurance challenges that are different to the challenges experienced with calcium napthenates as discussed above. It will be appreciated therefore that compositions and methods that may be suitable for inhibiting the formation of sodium carboxylate emulsions may not necessarily be suitable for inhibiting the formation of naphthenate solids.
Variations in observed water chemistry, pH, pressure, temperature and shear are generally accepted as the main factors affecting solids formation. As the pressure lowers, more carbon dioxide is lost from the hydrocarbon phase of the crude oil and the pH rises. This increases the degree of dissociation of the naphthenic acids leading to solids precipitation which accumulate at the oil-water interface, and as described above, may eventually block processing equipment and cause disruption to normal operation of the equipment.
The conventional method of preventing calcium naphthenate solids formation is the addition of relatively high amounts of organic acid. Various other chemical additives have been used to mitigate the formation of precipitates or emulsions in crude oil. For example, US 2005/0282711 A1 and US 2005/0282915 A1 (both to Ubbels et al.) disclose surfactant compositions containing hydrotopes such as mono- and diphosphate esters and methods for inhibiting the formation of naphthenate salts or emulsions. WO 2007/065107 A2 (Baker Hughes Inc.) discloses a method for inhibiting the formation of naphthenic acid solids or emulsions in crude oil in and/or downstream from an oil well.
However, there remains a need for alternative compositions which are more specific to inhibiting naphthenate solids formation from liquid hydrocarbons such as crude oil during extraction or separation processing steps for example, dehydration and gas removal.